Secondary threshold for 2015-16 and 2016-17 is £155 a week, not the statutory £156 #2178

Problem

gov.hmrc.national_insurance.class_1.thresholds.secondary_threshold holds £155 a week for 2015-16 and 2016-17. The statutory secondary threshold (ST) in both years was £156 a week, and £8,112 for an annual earnings period:

£155 was the primary threshold in those years (reg 10(c)). It looks like the PT figure was copied into the ST file.

I checked every other year from 2015-16 to 2025-26 against the legislation.gov.uk point-in-time text of regs 10 and 11. They all match: £157 (2017-18), £162, £166, £169, £170, £175 (2022-23 to 2024-25), then the annual £5,000 from 2025-26 (#1967, #2165).

Effect: employer NI in model years 2015 and 2016 is overstated by 13.8% × £52 = £7.18 for each liable employee earning over £8,112.

Coverage gap

test_ni_threshold_freeze.py pins the annual ST at £5,000 for 2025-2030, and its Hypothesis differential checks employer NI against the statute for 2026-2030. No test checks the annual ST in any year before 2025, and no property test checks employer NI in 2025-26 or earlier.

Fix

• Set the 2015-04-06 and 2016-04-06 values to 156, citing reg 10(d) and reg 11(3A)(b) at those dates.
• Add a statute table of the reg 11(3A)(b) annual ST and the secondary percentage for every tax year from 2015-16 to 2030-31. Test that 52 × the stored weekly ST equals the annual figure to the penny in every year. Test that, before 2025-26, the stored weekly value is the reg 10(d) figure itself.
• Add a Hypothesis differential: for any earnings, ni_class_1_employer equals rate × max(e − annual ST, 0) in exact arithmetic, for each of those years. 2022-23 is excluded, because the employer rate there is a separate open question (the 15.05% Health and Social Care Levy rate from 6 April to 5 November 2022 is not in the rate parameter).
• Add YAML cases for 2015 and 2016.